The present invention relates to an input screen for an image intensifier tube and a method of making the same.
Generally, an input screen for an image intensifier tube, such as an X-ray, a .gamma.-ray or other radiation ray image intensifier tube, is required to have a high resolution. Particularly a medical use image intensifier tube for taking a photograph of an organ of a human body is required to have such a characteristic. To improve the resolution, it is well known to have an input phosphor layer cracked in the direction of thickness to provide a kind of light guide. Such an input phosphor layer can be formed by vapor-depositing cesium iodide on a substrate having an uneven surface as described in, for example, U.S. Pat. No. 4,184,077. According to this patent, a surface of aluminum substrate is provided with fine grooves by anodizing, sealing and heat treatment. Phosphor blocks are then formed by depositing phosphor material on the surface of the aluminum substrate. Cracks in the phosphor layer are formed corresponding to the fine grooves. However, the islands separated by the cracks of the substrate have relatively large diameters of 50 .mu.m to 100 .mu.m and the phosphor blocks have similar diameters. These values are too large so that further improvement of resolution is required.
Recently an improved input screen has been developed and is described in U.S. application No. 272,764 filed on June 11, 1981 and issued as U.S. Pat. No. 4,437,011. This input screen has a first phosphor layer including phosphor crystal particles with a mean diameter of 15 .mu.m or less on a smooth surface of the substrate and a second phosphor layer formed on the first phosphor layer. The second phosphor layer includes individual columnar crystals grown on the phosphor crystal particles. This input screen improves resolution remarkably. However, the phosphor layer is formed on the even surface of the substrate and adhesion of the phosphor layer is weak. Therefore, strict control of the manufacturing process is needed to ensure adequate adhesion. As well known, when the phosphor material is vapor-deposited on the substrate at low temperature, the size of the columnar crystals is small and the resolution is improved, but adhesion becomes weak. On the contrary, when the substrate is high in temperature, the crystal spreads laterally on the substrate and adhesion increases. However, the resolution tends to decrease because of relatively large columnar crystals. Thus, strict control of the manufacturing process is required to obtain an input screen having both good adhesion and high resolution. This is difficult in mass production.
The present inventors investigated in detail the adhesion of a cesium iodide phosphor layer vapor-deposited on a smooth surface of an aluminum substrate. The phenomenon of peeling off of the phosphor layer was found to be a partial peeling off as plural cracks appear in one particular direction or the phosphor layer rose. The peeling off was particularly seen at the portion near the center of the substrate. Peeling off also occurs during the gradual cooling of the substrate after the vapor deposition of cesium iodide phosphor material. Thus, peeling off seems to be caused by the thermal expansion coefficient differential between aluminum and cesium iodide. The thermal expansion coefficient of aluminum is about 2.4.times.10.sup.-5 /.degree.C. at room temperature to 200.degree. C., and that of cesium iodide is about 5.3.times.10.sup.-5 /.degree.C. in the same range of temperature. Peeling off was particularly observed when an oxidized layer, such as Al.sub.2 O.sub.3, was formed on the surface of the substrate. The peeling off occured over a relatively large area even though it occured partially. Unevenness or scratches caused by the rolling and the structure of the substrate also encourage peeling of the phosphor layer. That is, when cesium iodide is deposited on the uneven or line-like scratched surface of the substrate, the phosphor layer is prone to peel off or to crack at uneven or scratched surface portions during cooling. If the substrate is made of a rolled sheet, the crystalline structure of the substrate has long crystal grains aligned along the rolling direction. Thermal expansion and thermal shrinking are larger in the direction along the longitudinal direction of the crystal grain than in the direction perpendicular to the longitudinal direction. During cooling after vapor-depositing, the aluminum substrate shrinks more in the longitudinal direction of the crystal grain than in other directions, so that the phosphor layer tends to crack or peel. It is practically impossible to avoid scratches or the unevenness caused by the rolling. It is also inevitable for the crystal grains to align along the rolling direction.